Logging measurement systems have been used for decades to measure various properties of earth formation penetrated by a well borehole. The first systems used downhole instruments or “tools” which were conveyed along the borehole by means of a “wireline” cable. In addition, the wireline served as a means of communication between the downhole tool and equipment at the surface, which typically processes measured data to obtain formation parameters of interest as a function of depth within the borehole. These measurements are commonly referred to as “well logs” or simply “logs”. Logging measurement systems can utilize nuclear, acoustic, electromagnetic, NMR and other types of measurements to obtain formation parameters of interest. For example, nuclear measurements can include measures of formation natural gamma radiation, thermal neutron flux, epithermal neutron flux elastic and inelastically scattered neutron, capture gamma radiation, scattered gamma radiation, and the like. A variety of formation parameters are obtained from these measurements, or combinations of these measurements, such as shale content, porosity, density, lithology and hydrocarbon saturation.
Wireline logging is applicable only after the borehole has been drilled. It was recognized in the 1960's that certain operational and economic advantages could be realized if drilling, borehole directional, and formation properties measurements could be made while the borehole is being drilled. This process is generally referred to as measurement-while-drilling (MWD) for real time drilling parameters such as weight on the drill bit, borehole direction, and the like. Formation property measurements made while drilling, such as formation density and formation porosity, are usually referred to as logging-while-drilling (LWD) measurements. The LWD measurements should conceptually be more accurate than their wireline counterparts. This is because the formation is less perturbed in the immediate vicinity of the borehole by the invasion of drilling fluids into the formation. This invasion alters the virgin state of the formation. This effect is particular detrimental to the more shallow depth of investigation nuclear logging measurements.
A brief summary of operating concepts of a nuclear density measurement system is presented so that the present invention can be more easily understood. The downhole instrument, or “tool”, comprises typically a source of radiation and one or more radiation sensors or “detectors” axially spaced from the radiation source. For purposes of discussion, it will be assumed that the tool comprises a single source that emits gamma radiation, and two gamma ray detectors that are disposed within the tool at two axial spacings from the source. Gamma radiation is emitted by the source, passes through any material between the tool and the borehole wall, and enters the formation where it interacts with material within the formation. A portion of the radiation is scattered back into the borehole at a reduced energy. A portion of radiation scattered back into the borehole is recorded by the gamma ray detectors. Source gamma ray energy is selected so that the primary mode of reaction is Compton scatter, which is related to the electron density of the composite formation material including the formation matrix material and any fluid filling pore space within the matrix. Electron density is, in turn, related to the “bulk” density of the formation. The count rates measured by each gamma ray detector can, therefore, be related to the formation property of interest, which is bulk density. These relationships are determined by calibrating the tool under known borehole and formation conditions with “fixed” axial source-detector positionings. Stated another way, the tool is calibrated assuming that the spacings between the source and each detector remains constant while operating in a borehole environment. Gamma radiation not only interacts with the formation, but also with any intervening material between the tool and the borehole wall. This intervening material includes borehole fluid and particulate material, known as “mudcake”, which builds up on the borehole wall due to invasion into the formation of borehole fluid. Mudcake and any other intervening material adversely affect the bulk density measurement. Responses of the two gamma ray detectors are combined to minimize the effects of mudcake and tool standoff. The “spine and rib” technique is known in the art as one method for combining the two detector responses.
A typical wireline scatter gamma ray density tool is constructed with a gamma ray source and one or more gamma ray detectors disposed within a “pad” which is mechanically forced against the borehole wall to minimize standoff effects. The pad is typically physically robust with minimal void space. Radical changes in pressure, which are typically encountered in a borehole environment, have minimal effect on the dimensions of the pad. As a result, source-detector geometry is relatively invariant to changes in borehole pressure. The response of the system, which is typically calibrated at atmospheric pressure with a fixed source-detector geometry, is typically minimally affected by large changes in borehole pressure.
An LWD scatter gamma ray density tool differs from its wireline counterpart in many aspects. One of the main differences stems from the fact that the source and detectors of a LWD scatter density tool are mounted in the drill collar rather than inside a wireline pressure housing, such as a pad. Such layout imposes certain restrictions on the size of the detectors, the length of a pressure housing containing the detectors, the robustness of the pressure housing, and the way the source, the detectors, and intervening radiation shielding are disposed in the collar. A typical layout of an LWD scatter density tool comprises a source shield, made of a heavy material such as tungsten, that is directly affixed to or fabricated as an integral part of the collar. The source shield typically comprises collimator passage openings or “window” covered with a light material relatively transparent to gamma radiation. Axially spaced detectors are typically disposed in a detector pressure housing, which is typically flexibly attached to the source housing. Stated another way, the source and detector housings are not rigidly attached to each other. Components within the detector pressure housing are at an ambient pressure, such as atmospheric pressure, at which the tool is calibrated. Detector shielding components are made of a heavy, gamma radiation absorbing material, such as tungsten. The shielding components also typically comprise collimator passages with windows covered with a light material relatively transparent to gamma radiation. The passages and windows are oriented in the pressure housing to collimate gamma radiation scattered from the borehole and formation environs. In one common embodiment, a stabilizer blade is then assembled over the source housing and the detector pressure housing. Under pressures encountered in the harsh borehole environment, the source remains in its original position since it is directly mounted to the collar. The detector pressure housing, however, compresses under this increased pressure. This compression changes the source-detector axial spacing from that at which the tool was calibrated. This change in source-detector spacings results in non-density related changes in count rate thereby yielding erroneous bulk density measurements.
Pressure related errors of type discussed above are typically more significant in LWD system than in wireline systems.